Two reason why a current flow in the forward bias condition
1) In the forward bias condition, the forward bias voltage applied to a diode overcomes the built-in potential barrier within the diode. This allows the majority charge carriers (electrons in N-type and holes in P-type) to easily flow across the junction and contribute to the current flow. Therefore, one reason for current flow in the forward bias condition is the reduced potential barrier.
2) The forward bias condition also aligns the energy levels of the conduction band in N-type material and the valence band in P-type material. This alignment creates a continuous energy path for charge carriers to flow through the diode, resulting in current flow. Hence, another reason for current flow in the forward bias condition is the establishment of a favorable energy path for charge carriers.